Radiation-curable compositions comprising a cationically polymerizable compound and a photoinitiator for cationic polymerization are well-known in the industry and are used, for example, as radiation-curable paints, photoresists or for the production of three-dimensional articles by stereolithography. The photoinitiator for the cationic polymerization is formed in these compositions by a strong latent acid, i.e. a compound which undergoes a photoreaction on irradiation to form a strong acid, which then initiates the cationic polymerization.
In a stereolithographic process, as described in greater detail in U.S. Pat. No. 4,575,330, which is incorporated herein by reference, three-dimensional articles are built up in layers from the radiation-curable composition by first irradiating a layer of the composition imagewise. The composition is irradiated either over the entire area or in a predetermined pattern (with linear or vectorial scanning) using a UV/VIS light source until the layer has solidified in a desired layer thickness in the irradiated areas. A new layer of the radiation-curable composition is then provided over the layer that has already been solidified. The new layer is similarly irradiated over the entire area or in a predetermined pattern forming a second solidified layer adhering to the first.
This layering and irradiating operation is continued so that repeated covering of the previously solidified material with new layers of curable composition and subsequent irradiation of the new layer produces a three-dimensional article, also known as the "green part". The so-called "green part" is not fully cured, but is sufficiently solidified to withstand its own weight. The green part is removed from the bath containing the radiation-curable composition and post-cured in a different way, such as by heat and/or further irradiation to produce a final cured article or product.
It is known that the addition of filler material, such as inorganic materials, ceramics, composites, metallic filler, organic polymeric material, glass, thermoplastics, silica beads, etc. to radiation-curable compositions improves most of the thermomechanical and mechanical properties of resulting cured articles. One of the significant drawbacks or limitations of filled resin systems in stereolithography systems is sedimentation of the filler material. Stereolithographic compositions are used for extended periods of times, upwards of up to a year. Over time, the filler material tends to settle to the bottom of the vat. As the filler settles, the viscosity and specific gravity of the filled composition gradually drops at the top of the vat where the part is built, and substantially increases at the bottom of the vat. In addition, the refractive index of the filled composition gradually changes depending on sedimentation rate; the change is significant for unstabilized compositions that show high sedimentation rate.
Consistent physical properties and accuracy are two of the most important properties for cured articles made in stereolithographic systems. However, multiple cured articles prepared over an extended period of time from an unstabilized filled composition exhibit different thermomechanical and mechanical properties. Additionally, changes in viscosity, specific gravity and the refractive index create a situation wherein the original building parameters of the originally homogenous filled composition are no longer valid. A requirement that the building parameters be constantly monitored and/or modified leads to a stereolithographic part building process that is problematic, inaccurate and unpredictable. Accordingly, one of the objectives of the present invention is to prevent filler sedimentation in stereolithographic process systems, and thereby ensure homogenous compositions and resulting cured articles.
It is well-known that inorganic or organic antisedimentation agents can be incorporated into filled resin compositions to reduce sedimentation. Organic antisedimentation agents have been used in the paint industry as thixotroping agents to provide antisettling and sag-resistance properties. The most significant organic antisedimentation agents are: 1) castor wax derivatives, 2) particles of synthetic polyamide waxes, 3) synthetic polyurethane, 4) a micronized hydrogenated castor oil or a polyamide modified micronized hydrogenated castor oil and 5) crosslinked carboxylvinyl polymer.
The organic antisedimentation agents include homopolymers of acrylic acid crosslinked with an allyl ether of pentaerythritol, an allyl ether of sucrose, or an allyl ether of propylene. The crosslinked acrylic acid is neutralized with a suitable alkali or amine. The particles of synthetic polyamide waxes, synthetic polyurethane, polyamide modified micronized hydrogenated castor oil and crosslinked carboxylvinyl polymer may not be used in stereolithographic filled compositions that are cured via a cationic photoinitiator due to the basic nature of said agents or neutralized forms of said agents. Said agents or neutralized forms of said agents reduce the photospeed of a composition. Castor wax products, on the other hand, may be too hydrophobic and exhibit phase separation within the filled composition.
Conventional inorganic antisedimentation agents, such as Aerosil and Cabosil type products, are acidic. Many antisedimentation agents and filler materials cause an undesired viscosity increase of the stereolithographic filled compositions during extended periods of time, and drastically, reduce the shelf-life of the filled composition. As described above, the continuous viscosity increase creates problems in part building.
It is known that the viscosity of stereolithographic compositions can be increased to as much as 50,000 cps. at part building temperatures (approximately 25 to 45.degree. C.) in order to support the filler material. However, a composition having such a high viscosity is not suitable for currently available stereolithography systems. A highly viscous composition is undesirable in stereolithographic systems due to the potential for catastrophic part failure and increased leveling time of the top liquid layer in the bath. The increased time for leveling can significantly reduce the productivity of a stereolithographic device.
A filled composition comprising at least one cationically and/or radically cured compound(s), at least one photoinitiator, at least one filler such as crystalline or amorphous silica, and at least one inorganic antisedimentation agent such as Aerosil.RTM. or Cabosil.RTM. shows unacceptably low viscosity stability for stereolithography applications. Without intending to be bound to any particular technical theory, the acidic nature of the hydroxyl groups of the filler and the antisedimentation agents are believed to contribute to the viscosity instability problem of the filled compositions. Hence, a filled composition that is stabilized against sedimentation by the addition of an inorganic antisedimentation agent, as described above, can not be useful for conventional stereolithographic applications. To date, the antisedimentation problem of stereolithography filled compositions has not been solved due to the inability of overcoming the viscosity instability problem.
The present invention overcomes sedimentation problems on stereolithography filled compositions without creating destabilization of the viscosity of the filled composition. The invention is particularly useful for filled compositions in stereolithographic systems.